The present invention relates to a current-balance switching regulator in which a plurality of switching power source portions are connected in parallel so that a stabilized voltage can be supplied to a load.
Switching regulators capable of supplying several thousands of amperes with just a few volts are in general structured by connecting in parallel a plurality of switching power source portions and are required to be arranged that each of the switching power source portions effectively conducts the load assignment.
For example, a current of substantially 2000A is required from a voltage of +5V in a large-sized computer system which supplies a load for which a stabilized voltage must be supplied. A switching regulator used for supplying such low voltages and high currents is in general structured by connecting in parallel a plurality of switching power source portions.
FIG. 4 illustrates a conventional switching regulator, in which, in a case where switching power source portions 41 to 44 are respectively capable of supplying a current of 500A with +5V, a current of 2000A can be supplied to a load 40 with +5V by connecting in parallel four switching power source portions 41 to 44. Referring to this drawing, reference numeral 45 represents an AC power source. When the switching power source portions 41 to 44 are connected in parallel as described above, the relationship between the output voltages V1 to V4 are, for example, determined to be V1&gt;V2&gt;V3&gt;V4. The voltage difference in this case is substantially several tens of millivolts.
Assuming that the overcurrent protection for each of the switching power source portions 41 to 44 is 500A the output voltage V1 from the switching power source portion 41 is highest in the case where a current of 500A or less passes through the load 40. Therefore, current I1 is solely supplied from this switching power source portion 41. In a case where a current of 1000A or less passes through the load 40, the overcurrent protection function of the switching power source portion 41 is activated, causing the output voltage V1 thereof to be lowered. As a result of this, a current I2 is also supplied from the switching power source portion 42 of the next output voltage V2.
Therefore, the relationship between the current and the voltage of the load becomes as shown in FIG. 5. Symbols OCP1 to OCP4 represent overcurrent protection operating values. Assuming that point A is an operating point, the voltage of the load 40 becomes a value between V3 and V4, the switching power source portions 41 and 42 supply the maximum rated current, the switching power source portion 43 supplies a fraction of the maximum rated current, and the switching power source portion 44 is brought to a state in which it does not supply any current.
As described above, in the case where a plurality of switching power source portions 41 to 44 are operated in parallel in accordance with the above-described conventional method (overflow method), when the load current exceeds the determined overcurrent protection current for the switching power source portion whose output voltage is set to a high value, the switching power source portion whose output voltage is arranged to be at the second level is also assigned to the load current. Consequently, the load assignment of the switching power source portion whose output voltage is arranged to be high becomes higher. As described above, in the switching power source portion supplying the determined overcurrent protection current and sharing a substantial load assignment, the internal loss becomes too great, causing the heating value to increase, and raising the temperature of the parts. As a result, the life becomes shortened. In particular, each life of parts such as a smoothing capacitor which comprises an electrolytic capacitor, a switching element such as field effect transistor, an output rectifying diode becomes shortened due to rise in temperature. On the other hand, when a forced cooling is performed, the bearing for the fan driving motor or the like becomes shortened due to the exhaust heat, causing a necessity for the same to be changed in a relatively short time period.
Furthermore, when a voltage regulation is performed, the switching power source portions 41 to 44 need to be respectively subjected to the voltage regulation. In addition, since the output current is supplied to the load 40 via the diode, it is substantially impossible for the output voltage of each of the switching power source portions 41 to 44 to be determined to the same level because of the deviations of the parts characteristics. As described above, there arises a problem that the change of the output voltage becomes too large in the whole due to the involved different in the output voltage of each of the switching power source portions 41 to 44.
A novel system capable of overcoming the above-described type of defects involved in the parallel operation of the switching power source portions in accordance with the above-described overflow method is disclosed in Japanese Patent Laid-Open No. 60-134771 (the application whose priority date is on the 4th, Nov., 1983 in accordance with the U.S. Pat. Ser. No. 549259, now U.S. Pat. No. 4,635,178). The content of the same will be summarized as follows.
FIG. 6 illustrates the novel system arranged in such a manner that, as an alternative to the parallel operation of a plurality of switching power source portions displaying a plain characteristic upon output voltage-current to the determined overcurrent protection current, a plurality of switching power source portions displaying a predetermined inclination characteristics upon current-output voltage are in parallel operated. As means for making the characteristic upon current-output voltage of each of the switching power source portions as described above, a circuit is employed comprises, as shown in FIG. 6, a shunt resistor Rs, a linear reduction offset amplifier A4, a current sensitive amplifier A3, potential division resistors R2 and R3, a transient current amplifier A5, a transistor Q1, a resistor RC, a referential amplifier A1, a programmed resistor Rp, and a precise reference voltage source 28. When the current passing through the shunt current Rs is changed from zero to level detected as the determined overcurrent protection current, the voltage generated at the shunt resistor Rs is amplified by the linear reduction offset amplifier A4 and the current sensitive amplifier A3, and the thus-amplified voltage is then applied to the current division resistors R2 and R3. At the intersection of these resistors, a voltage for reducing the current to be supplied to the positive input of the error amplifier A2 from the referential amplifier A is generated. That is, a voltage corresponding to the level of the current which passes through the shunt resistor Rs is generated at the intersection between the potential division resistors R2 and R3, and the current corresponding to the thus-generated voltage passes via the transistor Q1 and the resistor RC. Therefore, a current which decreases in accordance with increase in the load current is supplied, as an alternative to a constant current to the positive input of the error amplifier A2. As a result, when the current corresponding to the load current is introduced into the negative input of the error amplifier A2, the switching element disposed in a current converter/output filter is switched by this error amplifier 2, and the characteristic upon current-output voltage which outputs therefrom becomes the characteristic described above.
When a certain switching power source portion is intended to be shifted in the direction which burdens the load greater, the output voltage from its switching power source portion decreases due to its characteristics. It leads to a fact that the compulsory force to assign the load to the portion other than this switching power source portion is generated. Such operation is respectively generated between each of the switching power source portions. As a result, the switching control systems for the corresponding switching power source portions are, in the whole, structured to make the load assignment substantially uniform distribution.
In this type of conventional system (drooping system), unbalance generated at the time of load assignment of each of the switching power source portion is substantially overcome, and the defects arise in accordance with this can also be overcome.
However, in order to overcome the above-described objects, the nominal initial voltage level of each of the switching power source portions needs to be determined to be the same level. If they are not determined to be the same level, a tendency can appear that the load is concentrated at the switching power source portion which displays a high nominal initial voltage level. Similarly, if a difference lies in the inclination of the characteristic upon current-output voltage for each of the switching power source portion, an unbalance of the load assignment is also be generated. Therefore, the reference voltage source 28 needs to be made precise and stable. Since the characteristic upon current-output voltage involves the inclination, differing from the above-describe overflow method, the change in the output voltage in accordance with change in the load, that is, the deterioration in the precision of the output voltage is, involved to be generated.